High speed interfaces using double or quad transfer rates use clock and data signals that have controlled slew rates. Double Data Rate (“DDR”) devices may use a source-synchronous clocking protocol to transfer data from the memory to the memory controller. The devices also may be known as “doublepump” devices. DDR devices using double transfer rates transfer data to and from the memory using both edges of the data strobes. Drivers that use a push-pull design should have slew rates that are controlled carefully and symmetrically in both high-to-low (“HL”) and low-to-high (“LH”) transitions because data is being sent or received depending on these transitions. If the HL or LH transition times of the control signals significantly differ, the interface may no longer be operable. Thus, these slew rates should remain constant even with variation in process, temperature and voltage, and the slew rate should be calibrated for a given process, temperature and voltage. A slow slew rate may result in noise problems, while a fast slew rate may result in ringing within the interfaces. Signal integrity of the DDR interface may demand that the slew rate be carefully controlled to prevent noise problems and ringing.
Current calibration methods may not compensate symmetrically or adequately in technology where the push-pull driver devices that do not track directly with each other over process, temperature and voltage. For example, in some devices, the LH transition may be done by P-doped devices, while the HL transition may be done by N-doped devices.
In one known method for slew rate calibration an analog-to-digital (A/D) converter is used. This method makes use of a resistor and changes the drive strength of the transistor driving the resistor. The voltage across the resistor is monitored and when a certain voltage is achieved, the system knows that calibration has been reached. This method is not very dynamic and does not measure the actual slew rate directly, but a parameter that contributes to the slew rate.
Thus, a need has arisen in the art to measure slew rate directly without additional elements attached or incorporated into the drivers.